1. Field of the Invention
The present invention relates to an electrophotographic image forming device, such as a printer or a copier. More particularly, the present invention relates to a tandem color image forming device having a plurality of photosensitive bodies disposed in parallel.
2. Description of the Related Art
Generally, an electrophotographic tandem color image forming device is provided with a plurality of image forming units disposed in parallel. A plurality of photosensitive bodies form toner images representing yellow, magenta, cyan and black colors, respectively, by the corresponding image forming units. A transfer unit transfers the toner images formed on the photosensitive bodies onto a recording medium, such as a sheet of printing paper. A fusing unit fuses and fixes the toner images transferred onto the recording medium with heat and pressure.
Another tandem color image forming device further includes an intermediate transfer unit, such as a transfer belt or a transfer drum, disposed between the photosensitive bodies and the transfer unit. The toner images formed on the photosensitive bodies are not directly transferred to the recording medium, but are first transferred as a first transfer image onto the intermediate transfer unit and then retransferred from the intermediate transfer unit onto the recording medium.
These tandem color image forming devices having a plurality of photosensitive bodies that form a required image by simultaneously revolving the plurality of photosensitive bodies have an advantage that a required color image is obtained quickly. Comparatively, a color image forming device having a single photosensitive body therein forms a required image by revolving the single photosensitive body several times. However, since use of the plurality of photosensitive bodies and the image forming units has a complicated construction, it requires that the tandem color image forming device should be constructed to be easily assembled, maintained and repaired.
Accordingly, the tandem color image forming device generally does not configure the photosensitive bodies and the image forming units to be separated from one another, but to be contained in a plurality of image forming cartridges, each in which a photosensitive body and an image forming unit are modularized in a body, so that they may be mounted in or removed from main body as a single part as a cartridge unit.
Since the image forming cartridges are removably mounted in the main body, they provide an advantage that the parts therein may be easily maintained or repaired. However, because the photosensitive bodies in the respective image forming cartridges transfer corresponding color images onto the same image forming position of the recording medium or the intermediate transfer unit, color deviation may occur between the respective transferred color images if the photosensitive bodies are not arranged in parallel to one another when mounted in the main body.
To solve the problem, a conventional tandem color image forming device includes a shaft fixing apparatus 1 for fixing shafts of photosensitive bodies in image forming cartridges in the right position, as shown in FIG. 1.
The shaft fixing apparatus 1 includes first and second fixing parts 11 and 15.
The first fixing part 11 has a plurality of rotation supporting holes 13 to rotatably support first ends 22 of shafts 8 of photosensitive bodies 7 in a plurality of image forming cartridges 2.
The rotation supporting holes 13 are formed in a spaced-apart relation from one another at a cartridge frame 10 of the main body corresponding to the first ends 22 of the shafts 8 of the photosensitive bodies 7. Accordingly, the rotation supporting holes 13 receive and rotatably support the first ends 22 of the shafts 8 when the respective image forming cartridges 2 are mounted in the cartridge frame 10.
The second fixing part 15 has a plurality of rotating members 16 to rotatably support second ends 17 of the shafts 8 of the photosensitive bodies 7 in the image forming cartridges 2.
Each rotating member 16 is provided with a coupling axle 18, a support axle 33, and a driven gear 34. As shown in FIG. 3B, the coupling axle 18 has a triangle-shaped recess 18a formed at an end thereof. As shown in FIG. 3A, the recess 18a receives a triangle-shaped projection 17a formed on the second end 17 of the shaft 8 of each photosensitive body 7 when each image forming cartridge 2 is mounted in the cartridge frame 10. The support axle 33 is installed to be movable in a direction of arrows A and B in a support hole 10a formed in a spaced-apart relation from one another at the cartridge frame 10, and pressed in the direction of arrow B, that is, toward the second end 17 of the shaft 8, by a compression spring 21. The driven gear 34 is positioned between the support axle 33 and the coupling axle 18, and engaged with a gear train 19 for transmitting a driving force from a motor 30.
The gear train 19 includes a plurality of power transmitting gears 23 connected with a driving gear 20 formed on a driving axle 30a of the motor 30, and transmits the driving force of the motor to the driven gear 34 of each rotating member 16.
Operations of assembling and disassembling the image forming cartridges 2 of the conventional image forming device having the shaft fixing apparatus 1 constructed as described above is as follows.
Since the installation operation of all image forming cartridges 2 is the same, only the image forming cartridge 2, which is located at the lowest position, is explained.
First, the image forming cartridge 2 is inserted in the cartridge frame 10 of the main body, and placed on a supporting plate 36 of the cartridge frame 10.
Subsequently, the first end 22 of the shaft 8 of the photosensitive body 7 is arranged in line with the corresponding rotation supporting hole 13 by adjusting the position of the image forming cartridge 2, and is then inserted in the corresponding rotation supporting hole 13.
Then, as shown in FIG. 2, the rotating member 16 is pushed in the direction of arrow A against the elastic force of the compression spring 21, and moved toward the cartridge frame 10.
In this position, after the second end 17 of the shaft 8 is arranged in line with the coupling axle 18, the force of pushing the rotating member 16 in the direction of arrow A is released. Accordingly, the rotating member 16 moves in the direction of arrow B by the elastic force of the compression spring 21. As a result, as shown in FIG. 1, the end of the coupling axle 18 is maintained in contact with the second end 17 of the shaft 8. At this time, the projection 17a of the second end 17 is positioned in a coupled state that is inserted in the recess 18a of the coupling axle 18 or an uncoupled state that is not inserted in the recess 18a depending on the phase thereof.
When the motor 30 is driven to carry out the image forming operation, the driving gear 20 fixed on the driving axle 30a of the motor 30 drives the driven gear 34 of the rotating member 16 through the power transmitting gears 23 of the gear train 19. As a result, the coupling axle 18 of the rotating member 16 rotates in the corresponding direction.
As the coupling axle 18 rotates, the recess 18a of the coupling axle 18 rotates with respect to the projection 17a. At this time, in case that the recess 18a is in the uncoupled state with the projection 17a, when the recess 18a is in phase with the projection 17a, it is coupled with the projection 17a. 
As a result, the rotation force of the rotating member 16 is transmitted to the photosensitive body 7, and the rotation force transmitted to the photosensitive body 7 is again transmitted to other parts, such as a developing roller and the like, in the image forming cartridge 2 by a power transmitting mechanism (not shown), thereby the image forming operation being carried out.
However, according to the conventional shaft fixing apparatus 1 operated as above, to couple the second end 17 of the shaft 8 of the photosensitive body 7 to the rotating member 16 users have to arrange the second end 17 in line with the coupling axle 18 by adjusting the position or direction of the image forming cartridge 2 while pushing the rotating member 16 in the direction of arrow A. Since the arranging operation is carried out at a narrow space in the cartridge frame 10 of the main body, it is difficult and troublesome, and a large amount of assembling time is required, thereby productivity and assembling efficiency are deteriorated.
Further, the conventional shaft fixing apparatus 1 has the triangle-shaped projection 17a and the triangle-shaped recess 18a formed respectively on the second end 17 of the shaft 8 and the coupling axle 18 to serve as a coupling device for transmitting the driving force of the motor 30 to the shaft 8 of the photosensitive body 7.
Since with such a triangle-shaped construction, the projection 17a and the recess 18a require complicated machining in fabrication, they cause not only fabrication thereof to be difficult, but also increase fabrication costs.
Also, after fabrication or assembling, if the projection 17a is positioned in the uncoupled state that is not inserted in the recess 18a due to a difference in phase therebetween, it is apt not to slide into the recess 18a, but to idle by the rotating force of the coupling axle 18 in operation, thereby generating poor coupling.
Thus, if the projection 17a is not stably coupled with the recess 18a, the driving force of the motor 30 may not be transmitted well to the parts, such as the photosensitive body 7 and the developing roller contained in the image forming cartridge 2. As a result, the quality in the resultant image formed by the image forming cartridge 2 may be deteriorated.
Accordingly, a need exists for an image forming apparatus having an improved tandem color image forming device including a plurality of photosensitive bodies disposed in parallel that is easily inserted in and removed from the image forming apparatus, and providing uniform quality images.